Retinex processing is known as a method for compressing dynamic range through image processing. Retinex processing includes separating the original image into a reflected-light component and an illumination light component by using a low-pass filter, correcting the illumination light component, and outputting a combination of the corrected illumination light component and the reflected-light component.
While the edges are favorably reproduced when a small-sized low-pass filter is used, halo artifacts may occur around the edges when a large-sized low-pass filter is used.
Edge-preserving low-pass filters have been proposed to prevent halo artifacts, and ε filters are known as one type of such low-pass filters. For ε filters, a threshold ε is previously set, and of all pixels in a predetermined region, only pixels having values whose difference with that of the center pixel in the region falls within the threshold ε are used as the subject of calculation.
While lowering the threshold ε allows reducing halo artifacts, a Retinex processing effect, stereoscopic vision, is lost.
As seen above, it has been difficult to control halo artifacts to the extent that no halo artifacts can be visually recognized while maintaining low-pass filter effects.
Patent Document 1 discloses a technology which changes the threshold ε by using the edge strength in the target region. However, this technology only notes the edges around which halo artifacts may occur to determine the threshold ε.